A scintillator is a material, typically a crystal that responds to incident radiation by emitting light (e.g., a light pulse). Scintillators and compositions fabricated from scintillator materials are widely used in detectors for gamma-ray, X-rays, cosmic rays, and particles whose energy is of the order of 1 keV and greater. Often scintillators are used in X-ray imaging detectors for medical diagnostics, security inspection, industrial non-destructive evaluation (NDE), dosimetry, and high-energy physics. Using scintillator materials, it is possible to manufacture detectors in which the light emitted by the scintillator is coupled to a light-detection means and produces an electrical signal proportional to the amount of light received and to intensity of the incident radiation.
To acquire high resolution and quality picture, high dose radiation is required. Unfortunately, high dose radiation generally causes radiation damage to the scintillator material (i.e., the transmittance of scintillator crystal decreases with time), which inevitably leads to the loss of resolution and performance. One approach to solving this problem has been to lower the radiation dose. This approach requires better scintillators that provide higher light output to maintain the detector resolution and quality.
Recently, there has been an increasing demand for transparent, high atomic density, high speed and high light-output scintillator crystals and ceramic materials as detectors for computed X-ray tomography (CT) and other real time X-ray imaging systems. Many transparent ceramics like (Y,Gd)2O3:Eu3+, Gd2O2S:Pr, F, Ce, etc. have been developed for this purpose. Their slow response, however, and lack of single crystal form have limited their applications for X-ray explosive detection systems and X-ray panel displays.
Another approach to providing improved scintillator materials has been to improve the radiation hardness of the scintillator crystals so they can withstand high dose radiation. Scintillators typically used for x-ray explosive detection systems are mainly CsI and CdWO4 single crystals. Even though CsI provides a higher light output, its use has been problematic due to slow scan speeds associated with afterglow problems and low density for CsI,
CdWO4 crystals are more popular for X-ray explosive detection. CdWO4 possesses shorter afterglow times and demonstrates lower radiation damage than CsI. Unfortunately, the radiation damage of CdWO4 crystal also depends on the radiation dose. The higher the dose, the greater the radiation damage. For example, the radiation damage of CdWO4 crystal can reach as high as 40% when radiation dose is high enough.
Various approaches have been taken to reduce the radiation damage of tungstates. One approach has involved doping the scintillator material with other elements. Thus, for example, U.S. Patent Publication US 2003/0020044 A1, describes scintillator compositions formed from alkali and rare-earth tungstates, that have a general formula of embodiments, B is Gd3+ or Bi3+ and y is zero or ranges from about 0.0002 to about 0.001. In certain embodiments, x and y are both greater than zero. In certain embodiments, K1+ or Rb1+ and B is Bi3+ and x and y are both greater than zero. In certain embodiments, the doped cadmium tungstate is selected from the group consisting of Cd0.9995Na0.0005WO4, Cd0.9995Gd0.0005WO4, and Cd0.999 (KBi)0.001WO4.
In various embodiments this invention also provides a detector element for an x-ray detector (e.g. an element of an x-ray CT scanner). The detector element typically comprises a scintillator as described herein and, optionally, a photodetector (e.g., photodiode, photomultiplier, film, optical guide, etc.).
Also provided is a method of making a scintillator composition. The method typically involves combining essentially equal amounts of CdO and WO3 and minor amounts of a dopant that comprises at least one oxygen-containing compound of a monovalent metal selected from the group consisting of Li, Na, K, Rb, Cs, Ag and Tl, or/and at least one oxygen containing compound of a trivalent element selected from the group consisting of Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm and Yb, where the dopant(s) comprise less than about 0.1 mole percent of the amount of cadmium; and firing the mixture at a temperature and for a time sufficient to convert the mixture to a solid solution of cadmium tungstate. Another method of making a scintillator composition comprises preparing a solution of a cadmium compound, a tungsten compound, and an amount of a monovalent metal ion selected from the group consisting of Li, Na, K, Rb, Cs, Ag, T, and/or a trivalent metal ion selected from the group consisting of Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, where the monovalent ion and the trivalent ion when present are less than about 0.1 mole percent of the amount of cadmium; precipitating the compounds in a basic solution to obtain a mixture of oxygen-containing compounds; calcining the precipitate in an oxidizing atmosphere; and heating the precipitate at a temperature and for a time sufficient to convert the mixture to a solid solution of cadmium tungstate.
Also provided is an optical fiber comprising a scintillator composition of this invention where the optical fiber is optically coupled to the scintillator composition.
In still another embodiment this invention provides a method of producing an X-ray image. The method typically involves providing an x-ray detector comprising a AD(WO4)n. The composition CSY0.25Gd0.75W2O8 doped with Ca, showed improved radiation tolerance.
Another approach to improving radiation tolerance of scintillators has involved annealing the scintillator crystal in a controlled atmosphere. For example, the damage mechanism of PbWO4 has been analyzed, and it was found the damage was caused by oxygen vacancies. By annealing the PbWO4 crystal in an oxygen atmosphere, the defects in the crystal structure decreased significantly and the resistance to radiation damage improved.